Glass to metal seal for high-frequency electric discharge tubes



Aug. 29, 1950 -T TROMP 2,520,663

- GLASS T0 METAL. SEAL FOR HIGH-FREQUENCY ELECTRIC DISCHARGE TUBES Filed April 16, 1946 JHZQDOUBPH/ZMRTZROM INVENTOR.

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AT T 012N517 Patented Aug. 29, 1950 GLASS TO METAL SEAL FOR HIGH-FRE- QUENCY ELECTRIC DISCHARGE TUBES Theodoor Philibert Tromp, Eindhoven, Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application April 16, 1946, Serial No. 662,425 In the Netherlands April 6, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires April 6, 1963 3 Claims.

This invention relates generally to an electric discharge tube sealed by a plateor disk-like glass bottom and more particularly to theattachment of current-carrying conductors in this bottom.

In the case of conductors carrying high-frequency currents it is frequently desirable that these conductors should be, at least superficially, of good conducting metal, for example silver or copper. A suitable form of construction of such a conductor is constituted by a lead-through device which has a core of a material such as mlybdenum, tungsten or nickel-iron alloys, said materials having coefiicients of expansion that correspond substantially with the coefiicients of expansion of the glasses most commonly used in discharge tubes. This core is superficially coated with an extremely thin layer of copper or silver. There are, however, difliculties which arise if such conductors are to be sealed in a plateor disk-like glass bottom in a vacuum-tight manner. Specifically the outer layer of the conductor difiuses into the core metal, alloys with it, or vanishes in another manner as in the case of copper by oxidation or, as in the case of silver and. such materials having a low melting point due to the fact that this outer layer melts and flows off. This difficulty might not be expected directly, as the use of nickel-iron terminals coated with a copper layer, i. e. copper-sheath wire has been known for a very long time and their fusing-in does not entail great difliculty. A fundamental difference between such known seals and those which are referred to in connection with the present invention is that a well-known copper-sheath wire is sealed in a pinch, whereas a discharge tube according to the present invention has a plateor disk-like bottom. In sealin into a pinch the flames are directed on to the outside of the glass and the conductor to be sealed is protected from excessive heat by the intermediate glass. When conductors are fused into a flat bottom the flames are directed directly on to the seal or the studs are heated by conduction heating implements gripping the studs and the conductor itself is heated to a considerably higher extent than is required by the fusing-in operation; in addition, reheating of the leadthrough devices ensues when the bulb is sealed to the bottom. A further difference is that in connection with altering the coefiicient of expansion of the conductor to accord with that of the glass the copper or silver layer is generally thinner in the case of tubes having a plateor disk-like bottom than in the case of tubes having a glass pinch surrounding a copper-sheath wire. In tubes according to the invention, this layer thickness generally ranges between 5 and 10 microns, and in any case does not exceed 20 microns, whereas in the case of copper sheath wire it varies between 50 and microns in accordance with the total thickness of the conductor.

Owing to this difficulty encountered in scalingin, the ultimate conductor is not suitable for this purpose due to the lack of a proper superficial layer while on the other hand the deterioration of this superficial layer results in insuificient vacuumtightness. In addition, in either case the conductor has not the desired electrical properties.

These difficulties could be reduced by making the tube bottom of a glass having a low melting point but if used for very high frequencies these glasses entail excessive losses so that when the tube is in use the glass is overheated and finally the seal does not remain vacuum-tight.

These difficulties can be obviated completely by the use of an electric discharge tube according to the present invention which is closed by a plateor disk-like glass bottom having sealed in it one or more current-carrying conductors which are made of material readily secured in certain glasses, such as nickel, tungsten, molybdenum, alloys of iron and nickel, and which materials are coated with a layer of a metal having a low melting point and good conductivity, such for example as copper, silver or the like, and whose thickness does not exceed 20 microns. The conductors thus composed are secured in the glass bottom by means of an enamel whose melting point is lower than that of the glass.

Several advantages are ensured by the use of this combination. Thus, for example, the thin superficial layer of the conductor is prevented from being attacked or vanishing during the sealing-in operation. Secondly, use may be made of composite conductors formed by a mechanically strong core and a thin layer of a low melting point material of high-quality in so far as the conduction of high-frequency currents is concerned but being unsuitable by itself, on account of its low mechanical resistance and/or its coefficient of expansion, to constitute the conductor. Thirdly, the tube bottom may be made of the usual glasses, such as lead glass or the like, or glasses that may be harder and the choice of these glasses is not confined to those having a very low melting point. Copper and silver are among the metals that, as stated above, may be employed first and foremost for the superficial layer but other metals, for example magnesium, aluminium or the like, may also be used.

The enamel by means of which the sealing is efiected may be constituted by enamels known per se; when using lead or common glass (softening point 460 and 515 C. respectively) satisfactory results are achieved by means of a lead borate enamel composed of 65% by weight of PbO, 22.4% by weight of B203 and 12.6% by weight of SiOz (softening point about 400 C.) but use may also be made of other compositions such as a leadzinc borate enamel containing a considerable percentage of zinc oxide or else an enamel particularly suitable for use in combination with harder glasses and whose composition is: 17% of SiOz, 23% of B203, 25% of PbO, 10% of ZnO and 25% of MnOz (softening point 500 C.). For the attachment in the glass bottom the conductor is coated with a thin layer of the enamel and the sealing is then effected by comparatively low local heating, the melting of the enamel resulting in a vacuum-tight union between the glass bottom and the conductor. A temperature varying from 400 to 500 C. sufiices for this heating, whereas without the use of this enamel seal a temperature of at least 700 would be required, a temperature at which materials such as silver or the like melt away or oxidise to an appreciable and inadmis- .sible high extent, diffuse into the fundamental "metal or become alloyed with it.

It must be noted that the term enamel is to be understood to mean an inorganic vitreous mass by means of which a metal body is united with another body. Wherever the expression softenr ing point is used hereinbefore, it is intended to :mean the point at which a rod of the glass concerned having a length of 30 cms. and a section of 4 mms. and supported at both ends is given a sagging of 2 mms. by a weight of 195.5 grams.

In order that the invention may be more clearly understood and readily carried into efiect it will now be described more fully with reference to the accompanying drawing, in which part of the electric discharge tube according to the invention is shown.

Figure l is a view, partly in section, of a metal to glass seal in accordance with this invention;

and

It is obvious that the construction shown and the materials referred to constitute an embodiment of the tube according to the invention and that several other forms of construction and combinations of materials are possible.

What I claim is:

1. An electric discharge tube for high frequency operation comprising an envelope having a glass disc portion provided with a plurality of apertures, composite lead-through conductors within said apertures, each of said lead-through conductors comprising a ferro-chrome core having a metallic silver coating less than 20 microns thick thereon, and means comprising a thin enamel coating having a melting point less than 500 degrees Centigrade to seal said lead-through conductors in said apertures of said disc portion.

2. An electric discharge tube for high frequency operation comprising an envelope having a lead glass disc portion provided with a plurality of apertures, composite lead-through conductors within said apertures, each of said lead-through conductors comprising a ferro-chrome core having a metallic silver coating less than 20 microns thick thereon, and means comprising a thin lead borate enamel coating having a melting point less than 500 C. to seal said lead-through conductors in said apertures of said disc portion.

3. An electric discharge tube for high frequency operation comprising an envelope having a hard glass disc portion provided with a plurality of apertures, composite lead-through conductors within said apertures, each of said lead-through conductors comprising a ferro-chrome core having a metallic silver coating less than 20 microns thick thereon, and means comprising a thin leadzinc borate enamel coating having a melting point less than 500 C. to seal said lead-through conductors in said apertures of said disc portion.

THEODOOR PHILIBERT TROMP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,562,533 Weintraub Nov. 24, 1925 2,089,541 Dallenbach Aug. 10, 1937 2,100,187 I-Iandrek Nov. 23, 1937 2,193,640 Navias Mar. 12, 1940 2,334,020 Miller et al Nov. 9, 1943 FOREIGN PATENTS Number Country Date 15,342 Great Britain May 8, 1913 

